Fine, then running backs shouldn't be able to lower their head either. It just makes no sense...it either is or it is not okay to lower your head when initiating contact.
I have a lot of dislike for the targeting rule - especially how gray the conditions were at first (luckily, they've fixed a bit of that - but it still has its non-black-and-white aspects). At the same time, I understand why it's there - and this instance seemed pretty clear-cut to me, as much as I hated to see it.
That being said, ever since the targeting rule was implemented a few years back, the runner has often continued to do what runners have done for years and years - when a tackle is incoming, they get low. And that often means lowering their heads.
We see it with QBs and RBs the most, but also with WRs after making the catch and turning to advance the ball downfield. You try to get low (I assume "putting your head down and pushing through for extra yards" is the mindset that comes with it - although in some cases, it's just an instinctive reaction to an incoming hit).
So I agree with you wholeheartedly. Since day 1 of the targeting rule, I've been frustrated by this aspect of the game. If the rule is to avoid impact with the crown of the helmet for safety, you should bear some responsibility for moving your helmet into the path of an oncoming player and contributing to the impact.
That being said, that would introduce even more gray areas into an already muddy evaluation rule. I don't expect it to ever happen. IIWII.